Curse of Undeath

Death.

Almost every adventurer faces it at some point in their monster-slaying lives. From dragons to giants to beholders to goblins to orcs to kobolds to hobgoblins to bugbears to bandits to liches to skeletons to zombies to vampires to werewolves to banshees to drow to mind flayers to demons to devils to yugoloths to specters to wights to ghouls to ghasts to owlbears to ogres to umber hulks to bulettes to behirs to duregar to giant purple worms to ropers to cloakers to elementals to invisible stalkers to displacer beasts to formorians to trolls to ettins to yuan-ti to nagas to gith to the tarrasque, the fact of the matter is most creatures in the world want you dead.

Despite the entire ecosystem and then some wishing to put your body into the ground, many adventurers refuse to stay dead. Many are revived by the party cleric, some battle their way through the Nine Hells to return to their homes, and some simply return as spirits.

Not you.

A sorcerer that is touched with the curse of undeath find no rest in the world. For centuries after their deaths and their allies banding together to prevent the world-shaking BBEG from destroying the multiverse, for centuries after their friends have all passed onto the afterlife, sorcerers of undeath remain in the world, living their sorrowful existences. Mere reflections of their once living and happy selves, these spellcasters refuse to die not out of spite or willpower, but through the wicked magic that holds them, suspended in the Material Plane, to watch them suffer. 

At the DM's discretion, you may choose to take this subclass after your sorcerer of a different subclass is killed.

Rejuvenation

As a sorcerer of undeath, you are still susceptible to being killed, but you won't stay dead for long. Starting at 1st level, when you are killed, you will always be revived after 24 hours, even if your body is destroyed or if you are unwilling to return to life. When you are revived, your corpse will awaken at its current location or in a place of the DM's choice. The sorcerer will be revived at 1 HP and a -4 to all attack rolls, ability checks, and saving throws. After a long rest, the penalty will be reduced by +1.

Undead Attunement

At 1st level, your curse attunes your body to the necrotic energies of the Negative Plane. You gain resistance to necrotic damage. Additionally, you are able to sense the curse of the undead near you. As an action, you can detect the number and location of undead within 60 ft of you. When this occurs, you can learn one of the following traits of one of the revealed undead:

• One ability score of your choice
• Any damage resistances and immunities
• Any damage vulnerabilities
• Any condition immunities
• Any special senses
• If the undead can cast any spells

You can use this ability a number of times equal to your Charisma modifier

Deathly Sustenance

At 6th level, the fleeting souls of your foes can sustain you through your mortal wounds. Whenever you reduce a foe to 0 HP using a sorcerer spell, you gain temporary hit points equal to your sorcerer level + your Charisma modifier.

Trading of the Soul

At 14th level, you are able to harness the energy of your own soul to spare another's. You know the raise dead spell. Whenever you cast this spell, you do not require the material components to cast the spell, but casting the spell instantly kills you.

Impending Doom

At 18th level, you are familiar with your life force leaving your body, and can use your fleeting life force to your advantage. As a reaction, when you are reduced to 0 HP,  you are able to maintain a sense of consciousness. While you are dying, you are able to move, attack, cast spells, and take actions as if you are not unconscious for one minute. While you are in this state, you cannot benefit from any form of healing, you still fail saving throws when taking damage, and after one minute, you are instantly killed.